To this end, there have been proposed and implemented in the art a series of urea production processes essentially based on conversion reactions with differentiated yields in reaction spaces placed in parallel as described e.g. in European patent application EP-A-0 479 103.
In these processes, the total urea production is distributed between a main reaction space designed to cover the greater part of the required production capacity (generally from 60% to 80% thereof) and operating under low-yield conditions, and an auxiliary reaction space--so-called "once through"--operating under high-yield conditions and designed to bring production capacity to the final amount required.
Within the framework of the above mentioned processes, the reaction mixture from the main reaction space is subjected to a preliminary purification treatment to obtain a concentrated solution of carbamate recycled to the reaction space and a urea solution which is further processed and purified--together with the reaction mixture leaving the auxiliary reaction space--in a separation and recovery section.
From this latter section, a dilute carbamate solution and a substantially pure urea solution are obtained.
In accordance with the constant teaching of the prior art and for the purpose of increasing as much as possible the conversion yield in the auxiliary reaction space, this space is fed exclusively with substantially pure carbon dioxide and ammonia while the dilute carbamate solution is recycled exclusively to the main reaction space, to which is accordingly delegated the duty of converting into urea all the carbamate obtained in the purification sections located downstream thereof.
Although essentially meeting the above mentioned need, these processes exhibit both a weighted average yield limited by the rather poor yield in the main reactor and a series of plant limitations linked to the need of sending high recycle flowrates to the main reactor and provide an auxiliary reactor of large size and high cost.
The large quantity of water in the recycle solution to the main reaction space, furthermore, poses--despite the high conversion yield of the auxiliary reaction space--an upper limit to the average conversion yield achievable by the plant, which in turn limits the energy savings achievable in terms of high-pressure steam consumption reduction.
As disclosed in European patent applications EP 0 544 056 and EP 0 624 571, also known in the art are processes for the industrial synthesis of urea carried out by brand new plants wherein highly pure ammonia and carbon dioxide are reacted in a first reaction space, while a solution of recycled carbamate coming from an urea recovery section is sent to a second reaction space in which they react with an unreacted portion of ammonia and carbon dioxide.
Neither EP 0 544 056 nor EP 0 624 571, however, afford the problem of modernizing pre-existing urea production plants including a stripping equipment for treating the ureacontaining reaction mixture with the aim of enhancing production capacity while having at the same time a high weighted average yield.